In order to evaluate pistachio contamination to Aspergillus flavus in Semnan province; samples of pistachio kernels were collected from the last processing pistachio stage in traditional and semi-mechanized processing plants in 2010. They were examined for the presence A. flavus fungi. The samples were cultured on AFPA media. All plate were incubated for 3 to 7 days. At the end of incubation period, the plate were evaluated and isolates of A. flavus were identified and relative density of them were recorded. The isolates of A. flavus were evaluated for their ability to produce aflatoxins, when grown on natural substrate (autoclaved rice flour). The aflatoxins were extracted with chloroform and analyzed by TLC and densitometer. The results indicated that population density of A. flavus is varied with the type of processing plants. Traditional processing plants have more population density of A. flavus than semi-mechanized processing plants. 55 isolates of A. flavus out of the 86 tested isolates (%63.95) were able to produce one or several types of aflatoxins, while 31 isolates (%36.04) were unable to produce any type of aflatoxin. Among evaluated isolates, %9.30 of isolates produced all types of the aflatoxins, %8.13 produced AfB1,AfB2 and AfG1, %34.88 of isolates produced AfB1 and AfB2 and %11.62 of isolates only produced AfB1. Among aflatoxigenic isolates, the strength of toxin production varied from weak to strong.